cgen: fix fn mut argument of sumtype reference (#21874)

vlib/v/gen/c/fn.v

@@ -2608,8 +2608,14 @@ fn (mut g Gen) ref_or_deref_arg(arg ast.CallArg, expected_type ast.Type, lang as
 		g.write('&/*mut*/')
 	} else if arg.is_mut && arg_typ.is_ptr() && expected_type.is_ptr()
 		&& g.table.sym(arg_typ).kind == .struct_ && expected_type == arg_typ.ref() {
-		g.write('&/*mut*/')
-		g.expr(arg.expr)
+		if arg.expr is ast.PrefixExpr && arg.expr.op == .amp {
+			g.arg_no_auto_deref = true
+			g.expr(arg.expr)
+			g.arg_no_auto_deref = false
+		} else {
+			g.write('&/*mut*/')
+			g.expr(arg.expr)
+		}
 		return
 	} else if exp_is_ptr && !arg_is_ptr && !(arg_sym.kind == .alias
 		&& g.table.unaliased_type(arg_typ).is_pointer() && expected_type.is_pointer()) {

vlib/v/tests/fn_mut_arg_of_sumtype_ref_test.v

@@ -0,0 +1,29 @@
+module main
+
+pub struct Expr1 {}
+
+pub struct Expr2 {}
+
+pub type Expr = Expr1 | Expr2
+
+pub struct Gen {
+}
+
+fn (mut g Gen) foo(mut expr &Expr1) string {
+	return '${expr}'
+}
+
+pub fn (mut g Gen) bar(mut expr Expr) string {
+	return match mut expr {
+		Expr1 { g.foo(mut &expr) }
+		Expr2 { '' }
+	}
+}
+
+fn test_fn_mut_arg_of_sumtype_ref() {
+	mut expr := &Expr1{}
+	mut g := Gen{}
+	ret := g.bar(mut expr)
+	println(ret)
+	assert ret == 'Expr1{}'
+}